Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $28.42   8.720%   $30.8982   $30.90   $30.90   $30.90  
  Purchase #2 $220.96   8.345%   $239.3991   $239.40   $239.40   $239.40  
  Purchase #3 $34.98   9.687%   $38.3685   $38.37   $38.35   $38.40  
  Purchase #4 $195.60   7.458%   $210.1878   $210.19   $210.20   $210.20  
  Purchase #5 $169.52   12.142%   $190.1031   $190.10   $190.10   $190.10  
  Purchase #6 $60.66   6.555%   $64.6363   $64.64   $64.65   $64.60  
  Purchase #7 $115.75   9.142%   $126.3319   $126.33   $126.35   $126.30  
  7 purchase totals = $825.89       $899.9249   $899.93   $899.95   $899.90